Steam generators, especially the steam generators of pressurized-water nuclear reactors, have tubes bent in the form of a U, with the two legs of the U set at their ends in a tube plate. The curved parts of the tubes of the bundle which are located next to one another in its upper part have radii of curvature different from one another and are placed adjacent to one another to form a form substantially hemispherical form, dome.
During the operation of the steam generator, pressurized water at high temperature circulates in the tubes of the bundle and the feed water is brought in contact with the outer surface of these tubes, along which it travels in the vertical direction, thereby heating up and then evaporating, to emerge in the form of steam in the upper part of the generator.
The circulation of fluid in contact with the tubes can give rise to vibrations which are liable, to cause damage to these tubes if they are not held securely.
The straight part of the tubes is held securely by rigid elements located at uniform distances from one another over the height of the bundle. The curved parts of the tubes of the bundle forming the dome must also be held, and for this purpose spacer bars are usually inserted between the adjacent tubes of the bundle and arranged in substantially radial directions of the dome. These spacer bars are generally connected two by two in an articulated manner at one of their ends located towards the interior of the dome and are arranged angularly to form V-shaped structures.
The ends of the anti-vibration bars opposite their ends articulated on one another generally project relative to the tubes forming the outer layer of the dome.
Various means for fastening those ends of the anti-vibration bars to fastening elements located on the upper surface of the dome have been proposed.
It has been proposed, for example, to weld the ends of the anti-vibration bars to curved elements arranged in mid-planes of the dome.
It has also been proposed to fasten the ends of the anti-vibration bars, for example by welding, to substantially parallel rings resting on the upper surface of the dome.
At all events, the fastening of the anti-vibration bars requires complex operations usually involving welds which have to be made in the vicinity of the tubes of the bundle.
There are also known anti-flyoff devices for stabilizing the curved parts of the tubes of the bundle, which consist of yokes engaged on the last row of tubes and of which the ends projecting relative to the surface of the dome can be fastened by welding to elements for the retention of the anti-vibration bars, these elements being arranged, for example, in mid-planes.
Also known are structures for the retention of the tubes of a steam generator in the region of the dome, which consist of bows in the form of a portion of a ring surrounding the outer surface of the dome and arranged in mid-planes of the dome which all pass through the same diameter parallel to the rows of tubes. The bows have cutouts allowing them to match perfectly the upper form of the dome and are rigidly connected to one another by means of hoops perpendicular to the bows and screwed to these bows.
The assembly composed of the bows and of the hoops forms a structure which rests freely on the dome. This structure is retained on the dome under the effect of its own weight.
In some embodiments, holes are made in the bows in order to engage therein the anti-vibration bars which can be held by means of screwed connections.
This arrangement is unsatisfactory inasmuch the engagement of the anti-vibration bars into the bows is an operation which is difficult to carry out.